Particulate matter (PM) suspended in the atmosphere is a major pollutant affecting both human health and climate forcing. (see for example: Smith, K. R., Jerrett, M., Anderson, H. R., Burnett, R. T., Stone, V., Derwent, R., Atkinson, R. W., Cohen, A., Shonkoff, S. B., Krewski, D., Pope, C. A., Thun, N. J., Thurston, G., “Public Health Benefits of Strategies to Reduce Greenhouse-Gas Emissions: Health Implications of Short-Lived Greenhouse Pollutants”, The Lancet, 374:2091-2103, 2009; World Health Organization, “Review of Evidence on Health Aspects of Air Pollution—REVIHAAP Project: Final Technical Report” 2013; IPCC Fifth Assessment Synthesis Report, “Climate Change 2014, Synthesis Report”, 2014) There are a wide variety of PM sources and types, including wind-blown mineral dust from agricultural and mining activities, cement dust (calcium silicates and aluminates) from construction activity, fly ash (a coal combustion product containing sulfates and heavy metals), diesel exhaust and other products of incomplete hydrocarbon combustion, as well as other light absorbing organic matter. The latter types include organic carbon (OC), brown carbon (BrC), and black carbon (BC). (Andreae, M. O., and Gelencser, A., “Black Carbon or Brown Carbon? The Nature of Light-Absorbing Carbonaceous Aerosols”, Atmos. Chem. Phys., 6:3131-3148, 2006) A unifying terminology for carbonaceous aerosol information derived from optical absorption methods in the ultraviolet, visible, and near infrared wavelength regions is equivalent black carbon (EBC). (Petzoldl, A., Ogren, J. A., Fiebig, M., Laj, P., Li, S.-M., Baltensperger, U., Holzer-Popp, T., Kinne, S., Pappalardo, G., Sugimoto, N., Wehrli, C., Wiedensohler, A., Zhang, X.-Y., “Recommendations for Reporting ‘Black Carbon’ Measurements”, Atmos. Chem. Phys., 13:8365-8379, 2013)
Besides its particular composition, PM can be characterized by particle size or diameter. Inhalable particulates generally have diameters less than 10 μm (PM10) These include fine thoracic particulates with diameters less than 2.5 μm (PM2.5), which can penetrate into the trachea-bronchial and alveolar human respiratory regions and are therefore particularly unhealthy. Ultrafine respirable particulates (or “nanoparticles”) have diameters less than 100 nm (PM0.1) and can readily enter the circulatory system and then harm other organs.
While all types of particulates are considered to be serious health-threat pollutants, EBC particles that can be very accurately measured optically serve as a stronger indicator of harmful particle substances than does total PM. (World Health Organization, “Review of Evidence on Health Aspects of Air Pollution—REVIHAAP Project: Final Technical Report”, 2013; IPCC Fifth Assessment Synthesis Report, “Climate Change 2014, Synthesis Report”, 2014). Optical measurements of EBC can be performed with any desired particle size threshold (such as PM2.5). Optical determination of EBC has been studied extensively. (See for example: Moosmuller, H., Chakrabarty, R. K., Arnott, W. P., “Aerosol Light Absorption and Its Measurement: A Review” Journal of Quantitative Spectroscopy Radiative Transfer, 110:844-878, 2009; Lack, D. A., Moosmüller, H., McMeeking, G. R., Chakrabarty, R. K., Baumgardner, D., “Characterizing Elemental, Equivalent Black, and Refractory Black Carbon Aerosol Particles: A Review of Techniques, Their Limitations and Uncertainties”, Anal. Bioanal. Chem., 406:99-122, 2014); Bond, T. C., Anderson, T. L., Campbell, D., “Calibration and Intercomparison of Filter-Based Measurements of Visible Light Absorption by Aerosols”, Aerosol Science and Technology, 30:582-600, 1999; Arnott, W. P., Hamasha, K., Moosmuller, H., Sheridan P. J., Ogren, J. A., “Towards Aerosol Light-Absorption Measurements with a 7-Wavelength Aethalometer: Evaluation with a Photoacoustic Instrument and 3-Wavelength Nephelometer”, Aerosol Science and Technology, 39:17-29, 2005; Weingartner, E., Saathoff, H., Schnaiter, M., Streit, N., Bitnar, B. M., Baltensperger, U., “Absorption of Light by Soot Particles: Determination of the Absorption Coefficient by Means of Aethelometers”, J. of Aerosol Science 34:1445-1463, 2003; Hitzenberger, R., Jennings, S. G., Larson, S. M., Dillner, A., Cachier, H., Galambos, Z., Rouc, A., Spain, T. G., “Intercomparison of Measurement Methods for Black Carbon Aerosols”, Atmospheric Environment, 33:2823-2833, August 1999, and references cited therein.) In a common method to measure EBC optically, atmospheric aerosols are sampled onto filter media and the attenuation of light through the filter is monitored in real time as EBC-containing particulates accumulate. The change in optical attenuation over time is related to the accumulated quantity of EBC and the flow rate to yield a calculation of EBC concentration. Filter tape may be used in place of discrete filters to extend the period of unattended operation of the EBC monitoring instrument.
In U.S. Pat. No. 8,411,272 to Hansen, and as further explained in a later published paper (L. Drinovec, G. Močnik, P. Zotter, A. S. H. Prévôt, C. Ruckstuhl, E. Coz, M. Rupakheti, J. Sciare, T. Müller, A. Wiedensohler, A. D. A. Hansen, “The ‘dual-spot’ Aethalometer: an improved measurement of aerosol black carbon with real-time loading compensation”, Atmospheric Measurement Techniques, 8:1965-1979, 2015), the measurement accuracy of aerosol black carbon concentration by optical attenuation can be affected by filter tape loading effects, wherein the relationship between attenuation and accumulated EBC becomes nonlinear as attenuation values increase, especially as the filter nears saturation. In order to compensate for this effect, a dual-spot technique is used, wherein the aerosol in the same atmospheric volume is sampled at two different rates either by collecting the EBC sample through different filter areas or by passing the air through the filters at different flow rates, or by switching one or both flows on and off in rapid succession such that the time integrated flow, during the flow collection analytical period, differs between the two collected samples. The non-linear EBC density-attenuation relationship can then be characterized by combining two attenuation measurements. Thus, the compensation parameter can be determined from the actual measurement data instead of being predetermined using a priori assumptions that might not necessarily hold in the particular case, postdetermined (Virkkula, A., Mäkelä, T., Hillamo, R., Yli-Tuomi, T., Hirsikko, A., Hämeri, K., Koponen, I. K., “A Simple Procedure for Correcting Loading Effects of Aethalometer Data”, J. Air & Waste Management Assoc., 57:1214-1222, 2007) at each tape advance yielding only a temporally averaged compensation parameter over a collection analytical period, or postdetermined (Park, S. K., Hansen, A. D. A., Cho, S. Y., “Measurement of real time black carbon for investigating spot loading effects of Aethalometer data”, Atmospheric Environment, 44:1449-1455, 2010) based on long-term, over many collection analytical periods, statistical analysis yielding an even longer temporally averaged compensation parameter.
However, use of different filter areas or air flow rates can introduce systematic errors of their own. Using either different filter areas with the same flow rate or using different flow rates with the same filter area will produce differences in filter flow face velocities. Different filter flow face velocities can have different impacts upon particles of different sizes and different particle deposition depths in the sample filter, thereby, affecting measurement non-linearities. These can result in uncharacterized contributions to attenuation measurement differences from other than just the accumulation rate, and produce errors in the determined compensation parameter.
It is therefore desired that compensation for filter loading or other non-linear effects upon the measurement be determined without any change to the flow velocity (whether from different filter areas or flow rates) or any other variance that could differentially affect different particle sizes.